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Department of Physiology, University of Aarhus, Denmark
Prolonged or unaccustomed exercise leads to loss of contractility and muscle cell damage. The possible role of an increased uptake of Ca2+ in this was explored by examining how graded fatiguing stimulation, leading to a graded uptake of Ca2+, results in progressive loss of force, impairment of force recovery, and loss of cellular integrity. The latter is indicated by increased [14C]sucrose space and lactic acid dehydrogenase (LDH) release. Isolated rat extensor digitorum longus (EDL) muscles were allowed to contract isometrically using a fatiguing protocol with intermittent stimulation at 40 Hz. Force declined rapidly, reaching 11% of the initial level after 10 min and stayed low for up to 60 min. During the initial phase (2 min) of stimulation 45Ca uptake showed a 10-fold increase, followed by a 4- to 5-fold increase during the remaining period of stimulation. As the duration of stimulation increased, the muscles subsequently regained gradually less of their initial force. Following 30 or 60 min of stimulation, resting 45Ca uptake, [14C]sucrose space, and LDH release were increased 4- to 7-fold, 1.4- to 1.7-fold and 3- to 9-fold, respectively (P < 0.001). The contents of Ca2+ and Na+ were also increased (P < 0.01), a further indication of loss of cellular integrity. When fatigued at low [Ca2+]o (0.65 mM), force recovery was on average twofold higher than that of muscles fatigued at high [Ca2+]o (2.54 mM). Muscles showing the best force recovery also had a 41% lower total cellular Ca2+ content (P < 0.01). In conclusion, fatiguing stimulation leads to a progressive functional impairment and loss of plasma membrane integrity which seem to be related to an excitation-induced uptake of Ca2+. Mechanical strain on the muscle fibres does not seem a likely mechanism since very little force was developed beyond 10 min of stimulation.
(Received 27 April 2004;
accepted after revision 18 June 2004;
first published online 24 June 2004)
Corresponding author U. R. Mikkelsen: Department of Physiology, University of Aarhus, Ole Worms Allé 160, DK-8000 Århus C, Denmark. Email: urm{at}fi.au.dk
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